A control unit for a drive unit is described in German Patent No. DE 103 20 017 A1, which in particular controls or regulates the drive unit in regard to an output drive torque, the drive unit being an internal combustion engine of a motor vehicle. The motor vehicle typically includes a driver input transmission device actuatable by the driver of the motor vehicle, in particular a gas pedal actuatable using the foot. This is provided to output an output signal representing an instantaneous actuation state of the driver input transmission device. A control unit receives the output signal from the driver input transmission device and assigns the received output signal at least one setpoint output variable, in particular a setpoint drive torque of the drive unit. The drive unit is activated by the control unit in such a way that an actual output variable output by the drive unit approximates the setpoint output variable. Control units of this type are available in various designs for typical motor vehicle engines, in particular gasoline engines and diesel engines, e.g., Bosch engine-control systems having an electronic gas pedal (EGAS).
Furthermore, it is conventional to perform continuous torque monitoring to discover malfunctions in the vehicle control unit. This is used in particular to protect passengers in the motor vehicle and external traffic participants. Unintended acceleration of the vehicle is to be avoided. The core of continuous torque monitoring is a comparison of an actual torque provided by the engine to a permissible torque. In the normal case, the actual torque is less than the permissible torque. If the actual torque exceeds the permissible torque, an error exists in the engine control unit, and an error response resulting in a safer vehicle state is initiated. Monitoring of engine control units and also vehicle control units is typically performed according to a three-level monitoring concept. The engine control itself, in particular presetting the setpoint torque, is performed in the first level, referred to as the functional level. The second level (monitoring level) is implemented as continuous torque monitoring. In this level, a permissible torque is ascertained as a function of vehicle and engine functions, inter alia, and compared to an actual engine torque. The second level is made secure in a complex manner, e.g., by double saving of all variables, cyclic RAM and cyclic ROM testing, and by permanent program sequence control. A further, third level is used for computer security.
German Patent Application No. DE 197 39 565 A1 relates to a method for controlling the torque of a drive unit of a motor vehicle, in which the torque of the drive unit is set at least according to the driver input, the actual torque of the drive unit being determined and a maximum permissible torque being ascertained at least on the basis of the driver input. A torque reduction and/or torque limiting occurs if the maximum permissible torque is exceeded by the actual torque. At least one operating state is established in which the torque of the drive unit is increased due to additional load. During this at least one operating state, the maximum permissible torque is increased. In particular, the permissible torque is thus increased during operation with a cold drive unit and/or during operation of high-load consumers.
The above-described conventional methods for torque monitoring generally may not be transferred to hybrid vehicles without further measures. In hybrid vehicles, at least one further torque source (motor) is used in addition to an internal combustion engine. In most cases, it is an electric drive.
In the engine controller, the desired torque requested by the driver, which is set by operating the gas pedal, is distributed to the existing torque sources (at least two motors). This is performed as a function of numerous surroundings variables, inter alia, with the goal of setting the operating point which is most favorable for consumption for all torque sources, i.e., for all motors. Previously, however, no torque monitoring has been provided which deals with the special requirements of such a hybrid drive having one or more electric motors in addition to an internal combustion engine in the drivetrain of a vehicle. An additional electric motor connected to the drivetrain may cause an undesired vehicle acceleration at excessively high activation currents just like a “nonstop” internal combustion engine and thus also requires continuous torque monitoring.
In previous monitoring methods, monitoring has only been performed for excessively high torque. For motors which cannot generate a negative torque, this is sufficient, because in this way undesired acceleration of the vehicle may be reliably avoided. However, a new error source arises in systems having multiple motors, at least one of which may generate a negative torque. In these systems, thus, for example, hybrid drives, the vehicle may accelerate unintentionally although the total setpoint torque is not greater than the permissible torque. This may occur if a drive motor receives a setpoint value for a torque outside its settable range. Thus, for example, the vehicle battery may be charged during travel in hybrid vehicles. The at least one electric drive is to provide a negative torque of −100 Nm, for example, to charge the battery, so that the electric drive is operated in the generator mode in this case. To obtain a desired drive torque of 200 Nm, for example, the vehicle control unit increases the torque of the internal combustion engine by this amount, 300 Nm in the selected example. If the at least one electric drive is unable to provide the requested negative torque and may only output a negative torque of −10 Nm, for example, because of a temperature increase or overheating, an excessively high drive torque exists, 290 Nm in the present example, which significantly exceeds the desired drive torque of 200 Nm, for example. In previous designs of vehicle control units, this error would not be noted, because the total setpoint torque is correct and no feedback exists in a monitoring level of the vehicle control unit as to whether the requested torque, i.e., the desired drive torque, is settable. In previously selected approaches regarding this problem, a comparison is performed between the permissible torque and an actual torque output by the individual drives of the hybrid drive. However, this requires very great complexity, because the functional level of the actuators has had to be mapped in the monitoring unit of the vehicle controller.